High thermal stability and mechanical strength are much needed characteristics which make certain materials attractive candidates for applications such as heat engines, cutting tools, and turbine blades, articles which are presently made with expensive super alloys. Advanced ceramic materials are chemically inert compounds which have these desired characteristics. Current interest in these advanced ceramics centers around such materials as carbides, nitrides, borides, and silicides which have properties of hardness, corrosion resistance, and thermal stability that cannot be matched by metallic alloys or other structural materials. Examples of these ceramic materials are SiC, Si.sub.3 N.sub.4, TiC, TiN, VC, WC, and BN.
Although ceramic articles are valued for their extreme hardness, a major drawback has been their brittleness. Upon sufficient stress, a ceramic article will undergo catastrophic failure due to one or more cracks propagating through the material. Traditionally, ceramic whiskers have been used to reinforce composite ceramic articles. The whiskers, when incorporated into a ceramic matrix, add strength toughness to the resulting whisker-reinforced composite.
By dispersing whiskers or fibers into a ceramic matrix, a composite ceramic is formed which can resist cracking to a much larger degree than would be the case with just the ceramic material alone. As the density of whiskers is relatively high in these whisker reinforced ceramics, a crack starting in the material will probably encounter a whisker before it has a chance to propagate a significant distance. Ideally, when the crack reaches the whisker, it is deflected along the whisker-matrix interface instead of through the matrix, the result being that the crack is deflected instead of being allowed to propagate first through the whisker itself, and then through the remainder of the ceramic material. Because of this crack-deflecting property, a weak whisker-matrix interface actually helps to give the ceramic composite material increased strength.
Therefore, it is not desirable for the whiskers to be fused to the ceramic matrix since if they were fused, the whisker-matrix interface would be too strong and any crack started in the matrix material may continue through a cross-section of the whisker and subsequently the remainder of the ceramic matrix material. This undesirable situation will be evident upon examining the fractured surface of a composite material wherein the whiskers may be observed as having been cleaved along with the immediately surrounding ceramic matrix. Whisker pullout, on the other hand, occurs when a composite article with weak whisker-matrix interfaces is fractured and the whiskers protrude from the surface on one side of the fracture, while the complementary recesses remain where the ends of the whiskers previously resided in the matrix on the other side of the fracture. This pullout is strongly affected by the whisker-matrix interfacial shear strength.
Consequently, reducing the strength of the bond at the whisker-matrix interface in whisker reinforced ceramic composites encourages the deflection of any propagating cracks, thereby increasing the strength and toughness of the composite. Attempts at reducing the strength of the whisker-matrix interfaces have included reducing or eliminating the fusion of the whisker to the matrix. Examples of such attempts have included choosing optimum matrix and whisker combinations, since some combinations naturally tend not to fuse together as strongly as others. Other attempts have involved using whiskers which had higher coefficients of thermal expansion than those of the matrix. However, it was observed that this caused the whisker to pull away from the matrix as the article cooled after having been formed by firing. This pulling away from the matrix helped to keep the whisker and matrix from fusing.
Another method, which has been used in the past, included coating the whiskers, prior to dispersion into the matrix, with a material that would not fuse, or at least not fuse well, with the matrix. From these methods, one could see that an appropriate coating could lower the interfacial shear strength and also increase whisker pullout, resulting in greatly increased strength and toughness of the ceramic composite article.
Improved whisker-reinforced composite ceramic articles have been made by using whiskers which have been coated with one or more various ceramic or oxide coatings. The coatings were intended to prevent fusing of the whiskers to the matrix. These coatings, in addition, were sometimes intended to act as diffusion barriers for fiber-matrix combinations which would otherwise be incompatible since reactions between the materials could lead to fiber degradation. The methods used for applying these coatings to whiskers have been complicated, and often involved lengthy and/or expensive steps in their preparation. Another drawback was the limited types of coatings that any particular method could provide. Several such types of coatings were SiO.sub.2, TiO.sub.2, and AlO.sub.2 which could be coated onto mullite, alumina, and silicon nitride whiskers.
A certain degree of success has been experienced with whisker and fiber coatings fabricated by means of chemical vapor deposition as well as sublimation techniques. However, these processes, which can be somewhat complicated, result in nonuniform coatings due to their inability to properly expose all the surfaces of the whiskers during deposition.
Other coating processes which have been attempted have involved immersing carbon fibers or other substrates into solvents containing specific precursor materials, and then drying them in order to coat the fibers or substrates with the precursor material, which could then be subsequently processed into more stable ceramic or metal oxide coatings adhered to the surface of the fibers or substrate. These processes were limited, however, in that they could only coat one type of coating, for instance, either an oxide coating, or a ceramic coating, depending on the specific process.
Still other coating processes have involved immersing whiskers in a colloidal sol containing particles of oxides, and then drying the whiskers, leaving them coated with a particulate oxide coating. However, since the coating is particulate, and therefore is likely to form a non-continuous coating, the areas between individual particles left bare could serve as opportunistic sites for fusion between the matrix and the uncoated whisker surface. These particulate coatings have been used primarily to aid the dispersion of the whiskers into the matrix and not to inhibit fusion between the whisker and the matrix.
Yet still another prior art process involved incorporating whiskers into a matrix before the coating was actually completely formed. This meant that the introduction of the whiskers into the matrix material had to be done simultaneously with the final stages of the formation of the whisker coating. This requirement put undesirable constraints on both the whisker coating process and on the process of incorporating the whiskers into the matrix.
Examples of previous attempts to make useful coatings on ceramic whiskers are described in the following patents.
U.S. Pat. No. 4,806,428 issued to Cooper et al. on Feb. 2, 1989 and assigned to Corning Glass Works, Corning, N.Y., discloses a method for making a whisker-reinforced composite ceramic article which involves the steps, first, of forming a liquid sol or colloid comprising colloidal particles of one or more inorganic oxides. Generally, these oxides were selected from the group consisting of SiO.sub.2, Al.sub.2 O.sub.3, CaO, MgO, BaO, ZrO.sub.2, TiO.sub.2 and P.sub.2 O.sub.5. The sol was typically a water/alcohol-based sol wherein the oxide particles were generated by hydrolysis of alkoxides of the metallic constituents of these oxides. The sol was combined with an inorganic whisker reinforcement material, the material typically being selected from the group consisting of SiC, Si.sub.3 N.sub.4, C, Al.sub.2 O.sub.3 or a similar refractory inorganic whisker material. The combination was effected so as to form a uniform dispersion of the whiskers in the sol, which was separated into fine droplets and dried to form a dried particulate product comprising oxide encapsulated or oxide-coated whiskers. The oxide coating on the whiskers was made up principally of the oxide or mixture of oxides present in the initial fluid sol, and could be characterized as an assemblage of agglomerated, submicron-sized oxide particles forming a generally continuous coating over individual whiskers or whisker clusters. The oxide particles may be hydrous or hydrated oxides, and were generally, though not necessarily, present on the in an unreacted form, i.e. free oxide or hydrous oxide form.
U.S. Pat. No. 4,376,803 issued to Katzman on Mar. 15, 1983 and assigned to The Aerospace Corporation, Los Angeles, Calif. discloses a carbon fiber reinforced metal matrix composite produced by metal oxide coating the surface of the fibers by passing the fibers through an organometallic solution followed by pyrolysis or hydrolysis of the organometallic compounds. The metal oxide coated fibers so produced were wettable without degradation when immersed in a molten bath of the metal matrix material. Features of the invention include the use of metal oxide coatings to facilitate wetting of graphite fibers, and the use of alkoxide and organometallic solutions to deposit uniform metal oxide coatings on the surfaces of fibers.
U.S. Pat. No. 4,814,202 issued to Castelas on Mar. 21, 1989 and assigned to Centre Meridional d'Oenologie, Clermont-L'herault, France discloses processes for manufacturing thin membranes composed of an inorganic lattice of titanium and silicon oxides, wherein one or more titanium alkoxides and one or more silicon alkoxides are placed in solution in the same solvent, in order to obtain mixed titanium and silicon alkoxides. The mixed alkoxides were partially hydrolyzed by adding a basic aqueous solution having a pH of between 11 and 12 to apply a layer of hydrolyzed solution on a substrate. The particles in suspension were separated, the residual alkyl groups were eliminated by evaporation, and baking was effected at a temperature of between 700.degree. C. and 1250.degree. C. The processes according to the Castelas invention manufactured porous and/or semi-permeable membrane filter surfaces for filter, microfiltration, ultrafiltration or reverse osmosis cartridges.
U.S. Pat. No. 4,898,749 issued to Ritsko et al. on Feb. 6, 1990 and assigned to GTE Products Corporation, Stanford, Conn. discloses a method for producing aluminum oxide coated cobalt powder which comprises contacting cobalt powder of a fine particle size with a liquid aluminum compound wherein the aluminum is hydrolyzable, and adding water to the compound and the cobalt powder to form a slurry, removing essentially all of the liquid from the slurry to produce cobalt powder with a coating of hydrolyzed aluminum oxide, and firing the cobalt powder with the hydrolyzed aluminum oxide coating in a non-oxidizing atmosphere to produce cobalt powder having a coating of aluminum oxide.
U.S. Pat. No. 4,749,631 issued to Haluska et al. on Jun. 7, 1988 and assigned to Dow Corning Corporation, Midland, Mich. coats substrates by diluting a preceramic mixture of a partially hydrolyzed silicate ester in a solvent, the mixture then being applied to a substrate and ceramified by heating. One or more ceramic coatings containing silicon carbon, silicon nitrogen, or silicon carbon nitrogen can be applied over the ceramified SiO.sub.2 coating. A CVD or PECVD top coating can be applied for further protection. The invention was described as being particularly useful for coating electronic devices.
U.S. Pat. No. 4,911,992 issued to Haluska et al. on Mar. 27, 1988 and assigned to Dow Corning Corporation, Midland, Mich. coats substrates by diluting a platinum or rhodium catalyzed preceramic mixture of a hydrogen silsesquioxane resin in a solvent with a metal oxide precursor selected from the group consisting of an aluminum alkoxide, a titanium alkoxide, and a zirconium alkoxide. The preceramic mixture solvent solution is applied to a substrate and ceramified by heating. One or more ceramic coatings containing silicon carbon, silicon nitrogen, or silicon carbon nitrogen can be applied over the ceramified SiO.sub.2 /metal oxide coating. A CVD or PECVD top coating can be applied for further protection. The invention is particularly useful for coating electronic devices.
U.S. Pat. No. 4,801,510 issued to Mehrotra et al. on Jan. 3, 1989 and assigned to Kennametal Inc., Latrobe, Pa. discloses an article of manufacture having a SiC whisker reinforced alumina matrix substrate which has an alumina coating bonded to its exterior surface. The articles are described as having been found useful for cutting inserts in the high speed rough machining of steels.
U.S. Pat. No. 4,758,539 issued to Brown et al. discloses a process for preparing ceramic nitrides and carbonitrides in the form of very pure, fine particulate powder. An appropriate precursor is prepared by reacting a transition metal alkylamide with ammonia to produce a mixture of metal amide and metal imide in the form of an easily pyrolyzable precipitate.
Consequently, it would be desirable to have a process for efficiently coating ceramic whiskers with a uniform, continuous, non-particulate ceramic coating which would inhibit fusion between the whiskers and the matrix material when used in a whisker reinforced ceramic composite. It would also be desirable if the process was adapted to coat a wide selection of both oxide and non-oxide coatings, especially if the coating process could be completed on the whiskers before incorporating them into a whisker reinforced ceramic composite.
Therefore, it is a primary object of the present invention to provide a ceramic whisker coated with an oxide or non-oxide ceramic coating which is uniform, continuous, non-particulate and, when the whisker is incorporated into a whisker reinforced ceramic matrix, the ceramic coating inhibits fusing between the whisker and the matrix, yielding a strengthened composite.
It is yet another object of the present invention to provide an efficient method for applying both oxide and non-oxide ceramic coatings onto a ceramic whisker.
It is another object of the present invention to provide an efficient method for applying a uniform, continuous, non-particulate ceramic coating with a controllable thickness onto a ceramic whisker which, when incorporated into a whisker reinforced ceramic matrix, inhibits fusing between the whisker and the matrix, yielding a strengthened composite.